1. Field of the Invention
The present invention relates to a process of producing calcined materials for a ferrite magnet, whereby iron oxide which is inexpensive and obtained from mill scales and which has a coarse particle size is fired in an inclined rotary kiln, and more particularly to a process of producing calcined materials for a ferrite magnet, having good magnetic properties, the magnet being obtained by roughly milling the mill scales into a necessary particle size for primary oxidation treatment to change an oxidation percentage of the mill scales to 95% or higher, and after blending and mixing with oxide or carbonate of Sr or Ba, fired up to 700 .degree. C. by secondary oxidation treatment under a specific O.sub.2 content in an atmosphere at a charging side of raw materials of the inclined rotary kiln.
2. Description of the Prior Art
In general, a ferrite magnet (MO-nFe.sub.2 O.sub.3, M:Sr or Ba, n=5.0 to 6.2) is produced by calcining, milling, pressing and sintering, after mixing iron oxide obtained from mill scales composed of composition (wt %) shown in Table 1 with oxide or carbonate of Sr or Ba. In the calcining process, the iron oxide and oxide or carbonate of Sr or Ba are mixed and calcined in an inclined rotary kiln at 1300.degree. C. to 1350.degree. C. in air for ferrite formation.
Since oxygen in the kiln atmosphere is consumed during the ferrite formation at high temperatures in the inclined rotary kiln, at the charging side for raw materials the O.sub.2 content in the atmosphere is reduced to 5% or less. Thus after the primary oxidation treatment, the iron oxide obtained from the mill scales which is not completely oxidized is hardly oxidized more than in that state, and during the ferrite formation at the high temperatures, a foreign phase (Sr(Ba)O..sub.2 Fe.sub.2 O.sub.3) maybe produced in the calcined powders, with the magnetic properties of the finally obtained magnet being deteriorated.
TABLE 1 ______________________________________ T.Fe FeO Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 SiO.sub.2 Al.sub.2 O.sub.3 MgO CaO ______________________________________ 74.88 67.5 21.0 -- 0.25 0.15 0.005 0.20 ______________________________________
Since the mill scale contains a large amount of FeO besides Fe.sub.2 O.sub.3, the mill scale as a source of iron oxide must be oxidized completely in the atmosphere containing O.sub.2 or in air before mixing with the oxide or carbonate of Sr or Ba.
However, in order to oxidize a large amount of mill scales completely in the atmosphere containing O.sub.2 or in air first, the mean particle size of the mill scale must be made as fine as possible (12 .mu.m or less) before the oxidation treatment. Therefore, it takes a long time for milling, and moreover, fine powders less than 1 .mu.m are exhausted as dusts during the calcining; resulting in a high production cost and in a low productivity.
Therefore, the applicant has proposed a process of producing a ferrite magnet (Japanese Patent Application Laid Open No. Hei 3- 242908, U.S. Pat. No. 5,061,412), whereby mill scales milled into a specific particle size of 12 .mu.m or less are completely oxidized, until the Fe.sub.2 O.sub.3 content in the mill scales is raised to 98.0% or more, and mixed with a fixed quantity of SrO of BaO source, to obtain the ferrite magnet through the necessary treatment after calcining.
However, there was such a problem that, since the mill scales are hard and tough, it not only takes a long time to be milled below 12 .mu.m, but also coarse particles tend to remain in the fine particles. Therefore, in the case of calcining in the inclined rotary kiln at high temperatures of 1300.degree. C. to 1350.degree. C., an O.sub.2 content in the rotary kiln is consumed by the ferrite formation and the O.sub.2 content flowed through the charging side of raw material is reduced to cause deceleration of final oxidation of the mill scale. As a result, a foreign phase (Sr(Ba)O.2Fe.sub.2 O.sub.3) is produced in the calcined powders as previously mentioned, and deterioration and variations of magnetic properties of the finally obtained magnet are caused.